The effects of certain gastrointestinal hormones on gut blood flow and absorption will be determined. Blood flow at the gut absorptive site will be measured from the clearance of 3H2O and capillary pressure estimated from the arterial and venous pressure. The correlation of blood flow and pressure with unidirectional Na and H2O fluxes will be calculated to determine if a cause/effect relationship is present. The interactions between these hormones and the autonomic nervous system will be determined using appropriate blocking agents.